Project Information

Summary:

Weed population dynamics studies confirmed the weed suppressive effects of hairy vetch in cropping systems. Laboratory studies demonstrated that the effects were species dependent and strongly suggested allelopathy as a major mechanism involved. Under field conditions the inhibitory effect of hairy vetch residue was greatest during the first two weeks, suggesting that growers should avoid planting cash crop during that window of time. In addition to weed suppression, use of legume cover crops like hairy vetch can help reduce nitrogen fertilizer inputs in subsequent crops. This could help improve the long-term sustainability of the farm and reduce nutrient leaching.

Introduction:

Hairy vetch (Vicia villosa Roth) is a winter annual native to Europe and Asia. It is a winter hardy species that can accrue between 4.82-7.68 tons/ha in biomass, and it can contribute approximately 44.8 kg/ha of nitrogen (Ngouajio, personal communication) to the soil due to fixation (UC SAREP 2002). These characteristics along with its allelopathic potential against weeds make hairy vetch an ideal cover crop for regions with a temperate climate.

In 1989, White et al. found that hairy vetch aqueous extracts reduced corn and cotton germinations by up to 44 and 42%, respectively, depending on concentration. Corresponding radicle length reductions were 39 and 62%. In the same study, they found that the germination and radicle growth of pitted morning glory, wild mustard, and Italian ryegrass were all inhibited to some degree in the presence of hairy vetch extract. Screening tests run by Fujii in 2001 showed that water extracts of hairy vetch reduced radicle elongation in lettuce by 88% and hypocotyl growth by 11% compared to a non-treated control. However, germination was not affected. Using a methanol extract of hairy vetch radicle length was reduced by 82%, while hypocotyl growth was decreased by 48%. In this case, germination was decreased by 10% (he used 67g/L to extract for water and 250g/L for methanol).

In 1993, Hoffman et al. conducted a two year field study comparing various killing methods for hairy vetch prior to crop planting. The found that living and chopped hairy vetch reduced weed emergence and thus density, where as the rolled and glyphosate killed treatments did not. All treatments however, were found to reduce corn yield compared to the bare ground, weed-free control. Hairy vetch that is left living has been shown to suppress weeds longer than desiccated hairy vetch; however, weed densities in both were less than a bare ground control (Teasdale and Daughtry 1993). This finding suggests that something other than light transmission or temperature buffering is contributing to the reduction in weed density. More recently, in a two year study Ngouajio and Mennan (2005) reported reduced marketable cucumber yields in the presence of hairy vetch residues compared to a bare ground control during the second year. The yields in the hairy vetch plots during both years were significantly lower than those in the rye plots and sorghum sudan grass plots. This same study hairy vetch was shown to reduce weed density and biomass by 99 and 91%, respectively, compared to the bare ground control.

In cover crops that accrue a lot of biomass, competition appears to play a large role in reducing weed populations. However, the amount of biomass accumulated by hairy vetch does not account for the degree of weed reduction observed, perhaps further evidence of allelopathy (Fujii 2001). Drought conditions have been shown to exacerbate the growth inhibition caused by hairy vetch (Hoffman et al. 1993; Ngouajio and Mennan 2005). Therefore, it follows that the responsible allelochemicals are likely water soluble, resulting in higher concentrations under conditions, such as drought, that reduce leaching.

The allelopathic effects of the cover crops previously discussed are all at varying stages of research and development. Some such as rye and sorghum, have already had their allelochemicals isolated and identified and are looking at how to put that knowledge to use. Others such as hairy vetch are at the beginning stages of collecting evidence to support claims of allelopathy. More information is necessary prior to attempting to identify and understand the allelochemicals and their interactions in hairy vetch (Dakshini et al. 1999).

Project Objectives:

Understanding the allelopathic effects of hairy vetch on vegetables and weeds could help select the appropriate cover crop and crop rotation. The subsequent works examine the use of hairy vetch improve weed management and the sustainability of vegetable production systems. The objectives of these studies are as follows:

Objective 1. Study of the response of weed populations to hairy vetch residue.

Objective 2. Determination of the role of allelopathy in the weed suppressiveness of hairy vetch.

Objective 3. Measurement of the effects of timing of cucumber planting after hairy vetch kill on yield.

Research

Materials and methods:

Field Experiments

Hairy vetch was planted at the Michigan State Horticultural Research Farm in East Lansing, Michigan on September 2, 2003 and September 13, 2004. The plot was previously fallow in spring and summer of 2003. The experimental plot was divided into eight equal sized regions (each 215 m2), four of which were seeded at a rate of 39.2 kg ha-1 of hairy vetch and four of which were left fallow. The following springs, on May 28, 2004 and June 1, 2005, the whole field was disked. Caution was taken to avoid transferring hairy vetch residues to fallow areas.

A split-plot design with four replicates was used. Levels of the main plot factor were hairy vetch and bare ground. Each main plot was subdivided into six subplots, each randomly assigned to the six cucumber planting dates. Therefore, each hairy vetch treatment had a corresponding bare ground treatment to factor out environmental differences occurring over the six week planting period. Pickling cucumber ‘Vlaspik’ was planted at weekly intervals, 0, 1, 2, 3, 4, and 5 weeks after hairy vetch kill (WAK). Individual subplots consisted of four 9.14 m rows in 2004 and six 6.10 m rows in 2005. In both years, rows were spaced 46 cm apart, with an in row spacing of 13 cm. Prior to planting each week, the seed bed in each plot to be planted was prepared using hoes and rakes in 2004 and a rototiller in 2005. Two seeds per hole were planted by hand, as planting equipment was too large to fit in the individual plots. Three weeks after planting, cucumbers were thinned to one plant per 13 cm within the rows.

Three weeks after planting, in 2004, the control and hairy vetch plots received 448 and 336 kg ha-1 of a 34-0-0 (N2-P2O5-K2O) fertilizer, respectively. On account of symbiotic nitrogen fixation, the hairy vetch plots required less fertilizer. Previous studies showed that hairy vetch can add approximately 44.8 kg ha-1 of nitrogen in monoculture. In 2005, a 19-19-19 (N2-P2O5-K2O) fertilizer was applied to the entire field at 448 kg ha-1 on April 29 (i.e. 34 days prior to hairy vetch kill). At the time of cucumber planting, 198 kg ha-1 for the control and 119 kg ha-1 for the hairy vetch plots of 34-0-0 (N2-P2O5-K2O) fertilizer was added to total the same amount of nitrogen that was applied in 2004.

In the summer 2004, rainfall was sufficient to forgo irrigation. However, in 2005, sprinklers were used to supplement rainfall. Plots were weeded by hand as necessary leaving a 50-by-50 cm fixed-plot undisturbed for weed sampling. Subsequent hand weeding was performed as necessary.

During the cucumber season, all weeds in the 50-by-50 cm microplots were removed at three and six weeks after planting (WAP). These weeds were separated by species and counted. Due to the small quantity of most weed species, all weeds were recombined and dried at 70 °C for 7 days to obtain a dry biomass. To compare the weed biomass in the hairy vetch and bare ground plots throughout the season the 3 and 6 WAP samples were combined for each cucumber planting date.

Three weeks after planting cucumber stand was recorded. At harvest, a total of 60 feet of cucumbers were harvested (i.e. 2 rows in 2004 and 3 rows in 2005) from the center rows. Stand was recorded in the field along with total vine fresh biomass after the fruits were removed. Fruits were sorted into grades 1, 2, 3 (USDA 1997), and over-sized, then counted and weighed by grade.

Laboratory Bioassay Experiments

Plant Material Extraction. Hairy vetch was planted on September 3, 2003 and harvested on May 12, 2004 at the Horticulture Research and Teaching Center, Michigan State University, East Lansing, MI. No fertilizers, irrigation, or inoculums were used during the growth of hairy vetch. The area harvested was recorded in order to calculate biomass and extract production per unit area. All plant material was rinsed with reverse osmosis (RO) water and allowed to air dry prior to being weighed. The fresh plant material (25.49 kg) were then chopped by hand and blended with 2.3 L∙kg-1 of RO water, in an industrial blender for 30 to 60 sec. The pure blend was filtered through cheese cloth resulting in an average of 2.7 L∙kg-1. After centrifugation at 10,000 rpm for 10 min, the resulting supernatants were the desired hairy vetch aqueous extracts. Extracts were freeze dried using a tray-lyophilizer at 10 °C and the resulting powders were mixed to allow for uniformity. The powders were then stored at -20 °C until use in the bioassays. The residues remaining after water extraction were frozen at -20 °C until organic solvent extraction. The frozen residue of each plant was placed into an 8 L column, which was plugged above the stopcock with 4 layers of cheese cloth filled with cotton batting, and then filled with 4-5 L of methanol and allowed to stand at room temperature for a minimum of 24 h. The column was then drained and the extraction was repeated twice. The combined extracts were evaporated to dryness using a rotary evaporator at 32 °C. The resulting solid was the desired methanol extract, which was stored at -20 °C until use in the bioassays. The residue from the methanol extraction was further extracted with 4-5 L of ethyl acetate three times. The solids resulting from the ethyl acetate extraction were also stored at -20 °C.

Germination and Radicle Elongation Assays

The three extracts were used in bioassays to test the susceptibility of various vegetable crops and weeds species. The water extract was dissolved in RO water and the methanol and ethyl acetate extracts were dissolved in methanol, affording concentrations of 0, 0.25, 0.50, 1.00, 2.00, 4.00, and 8.00 g/L by serially diluting a stock solution of 10 g/L. These treatments were then applied to 90 mm Whatman No.1 filter paper in 100 mm plastic Petri dishes at 2.5 ml per dish (3.0 ml per dish for corn). The methanol was allowed to evaporate prior to seed placement, leaving behind the methanol soluble extract. The seeds of each species tested were sterilized in a 1% sodium hypochlorite solution for 10 min. They were then rinsed three times using RO water and placed 10 at a time on the dried filter papers. The weed seeds were soaked in RO water for 24 h after sterilization to increase germination rates. Once the seeds were in place, each Petri dish received about 2.5 ml of water. Petri dishes were subsequently sealed using Parafilm® and incubated in the dark for 4 to 11 days at temperatures specified in Table 1. After the incubation period, germination percentages and radicle length were recorded.

Research results and discussion:

Objective 1. Study of the response of weed populations to hairy vetch residue.

Overall, total weed biomass was not significantly different between the hairy vetch and bare ground treatments. Quackgrass (Elytrigia repens) and common purslane (Portulaca oleracea) were the most prominent weed species in both years. Quackgrass was inhibited by the hairy vetch treatments during three of the four weed sampling dates over the two years. Common purslane growth; however, was enhanced in the presence of the hairy vetch residues. This study suggests that hairy vetch alone is not sufficient to achieve desired weed suppression. Other strategies should be combined with a hairy vetch cover crop to improve weed management. Quackgrass and common purslane were by far the most prominent weeds during the two years of study. Of the weeds sampled, quackgrass seems to be the most sensitive species to hairy vetch. Based on observations prior to hairy vetch kill however, it is uncertain whether the suppression of quackgrass is due to allelopathy, competition during the off season, or a combination of the two factors (Wu et al. 2001).

Common purslane occurred at greater densities in the hairy vetch plots than in the bare ground plots, though only significantly so 3 weeks after planting in 2004. Perhaps increased moisture retention caused by the hairy vetch residues resulted in favorable conditions for common purslane growth (Teasdale and Mohler 1993, Teasdale and Daughtry 1993). Mohler and Teasdale (1993) observed increased emergence in some weed species in the presence of low rates of hairy vetch residues. Another possibility is that a compound released from the hairy vetch is stimulating the germination of common purslane, as ethylene has been shown to do to witchweed (Striga asiatica) (Putnam 1988).

Weed biomass was almost doubled from 2004 to 2005. Overall, hairy vetch tended to have higher weed dry biomass than the controls at the sample dates. Due its high density, common purslane contributed significantly to the observed total weed biomass. Future studies should examine weed biomass per species to obtain a better understanding of distribution. It appears that the best cucumber planting time to reduce weed biomass compared to the bare ground system would be approximately 2 to 3 weeks after hairy vetch kill.

Objective 2. Determination of the role of allelopathy in the weed suppressiveness of hairy vetch.

All extracts of hairy vetch (water, methanol, and ethyl acetate) inhibited weeds and vegetables growth in bioassay tests. The responses were plant species dependant and generally increased with increasing concentration of the extracts. Low concentrations of water extract stimulated the radicle growth of carrot, pepper, barnyardgrass, common milkweed, and velvetleaf. At higher concentrations all species tested were negatively affected by water extracts. The order of species sensitivity as determined by the IC50 (concentration required to produce 50% radicle inhibition) values, was common chickweed > redroot pigweed> barnyardgrass E1 > carrot E1 > wild carrot > corn > carrot E2 > lettuce > common milkweed > tomato > onion > barnyardgrass E2 > velvetleaf > pepper > cucumber (most sensitive to least sensitive). Using methanol and ethyl acetate extracts germination was significantly reduced except for corn and tomato. The radicle growth of most species, with the exception of corn and cucumber, was reduced by both organic extracts. Corn and cucumber radicle elongation was stimulated at low concentrations of the extracts; however these observations were not significantly different among treatments. This study demonstrated that methanol and ethyl acetate extracts of hairy vetch and cowpea contained allelopathic compounds and that their phytotoxicity is likely species specific (Hill et al. 2006, 2007).

Objective 3. Measurement of the effects of timing of cucumber planting after hairy vetch kill on yield.

Vine biomass per plant was increased for the latest planting dates. Fruit number was not significantly affected among planting dates Yield reached two peaks during the season at the 0 and 4 WAK (weeks after hairy vetch kill) planting dates. The trend observed indicates that any allelochemicals released are present between 1 and 3 WAK. All yields were higher in the hairy vetch treatments than the bare ground. To achieve the highest yields and avoid difficulties planting in heavy residues, waiting 3-4 WAK would be the best planting time for cucumber. When comparing the stand counts taken at harvest to those 3 WAP, it was found that some planting dates resulted in reduced stands in the hairy vetch plots and others in the control plots. Based on this observation, hairy vetch does not seem to significantly impact stand between these times. This implies that any potential allelochemical from hairy vetch may be reducing cucumber growth as opposed to killing the seedling. It is also interesting to note that some of the highest stands at harvest equated to some of the lowest yields. Therefore, yield per plant was low at that time; this could be related to allelopathic interference, or simply to intraspecific competition.

Vine biomass per plant, both fresh and dry, increased with delayed planting date in the hairy vetch plots. Perhaps the increase in biomass toward the end of the season indicates a release from allelopathic pressure or a better synchrony between nutrient release from the hairy vetch residue and cucumber uptake.

When looking at cucumber yield, the hairy vetch plots planted at 0 WAK and 4 WAK produced the highest yields. The trend seems to indicate that the allelochemicals from hairy vetch are not released immediately or readily available after incorporation (Inderjit et al. 1995), but rather during the 1-3 WAK period. It also suggests that once plants are established their yield is not affected by these allelochemicals (e.g. the 0 WAK plants were able to grow sufficiently so that when the allelochemical concentration increased during hairy vetch decomposition they were no longer susceptible). Though the 0 and 4 WAK plantings performed the best among hairy vetch plantings, it is important to note that all planting dates within the hairy vetch plots yielded above their corresponding bare ground plots. This cannot be attributed entirely to nitrogen fixation by hairy vetch since the fertilizer rates were reduced in the hairy vetch treatments unless hairy vetch was contributing more than our estimated 44.8 kg ha-1. Teasdale and Shirley (1998) found hairy vetch residues occasionally could add over 112 kg ha-1 of nitrogen during the growing season. Hairy vetch plots were ready to harvest up to a week ahead of their bare ground counter parts. This suggests that better growing conditions existed in the hairy vetch system. Several other benefits of cover crops including temperature buffering, moisture retention, and increased beneficial organisms are documented in the literature (Teasdale and Daughtry 1993).

3. Ngouajio M. 2005. Manage soil, manage weeds. The Organic Report. American Vegetable Grower. August 2005 P4. Note Permission was granted to the following Magazine to publish this article: (1) American Fruit Growers (Published in the August 2005 Issue), (2) Western Fruit Growers (Published in the August 2005 Issue), (3) Florida Grower (No feedback on publication process), and (4) Productores de Hortalizas (permission granted to Ana Reho, Managing Editor to translate and publish the article in Spanish)

Over 15 extension presentations at the local, regional, national and international levels.

Project Outcomes

Project outcomes:

Our study has demonstrated a significant benefit of incorporating hairy vetch into pickling cucumber cropping system. Cucumber yield was consistently greater in the hairy vetch treatment compared to the bare ground treatment as a result of better nutrient provisioning.

Our results have clearly shown that by using cover crops, vegetable growers can significantly reduce weed growth between cash crop seasons. This could help reduce the amount of weed seeds in the soil and ultimately reduce weed populations in subsequent crops.

In two year of study, the effect of hairy vetch on weed populations was species dependent. Quackgrass, a perennial and troublesome weed was effectively suppressed by hairy vetch. However, populations of other species like common purslane were not affected. This mean that the cover crops should be used in combination with other weed management tools to achieve maximum benefits.

Our findings were presented at the Great Lakes Fruits, Vegetables and Farm Market Expo in 2005 and 2006, American Society for Horticultural Sciences meeting in 2005, Weed Science Society of America Annual meeting in 2005, North Central Weed Science Society meeting in 2004 and 2005, and at various growers meetings. Though extension activities and publications we disseminated our results to over 2,500 growers, extension educators, weed scientists and horticulturists.

Economic Analysis

Not applicable

Farmer Adoption

The major practical result from our study was the determination of the period that growers should observe between hairy vetch kill and crop planting. We estimated from our data that a minimum of two weeks was required depending on location and rainfall. This finding has been presented to and well received by growers in Michigan and throughout the region throughout extension efforts. Many growers have also indicated that they are following the recommendation for other cover crops like cereal rye, and mustard species.

Recommendations:

Areas needing additional study

This study allowed improving our understanding of some of the processes underlying the weed suppressive effects of hairy vetch cover crop. While the potential allelopathic effects of hairy vetch were strongly suggested by our results, several additional steps are needed to provide a definitive confirmation.

These include:

1. Isolation and identification of the allelochemical(s) found in the water, methanol and ethyl acetate extracts of the hairy vetch residues.

2. Test of the activity of the pure products isolated from the cover crop residue.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.

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